The present invention relates to a combined distance and tilt sensing apparatus which is useful in the microlithographic forming of circuit patterns on the surface of a semiconductor wafer during the manufacture of integrated circuits.
Microlithographic techniques are widely used for forming circuit patterns on a resist-coated semiconductor wafer. As circuit details become increasingly smaller in dimension, more stringent requirements are placed on the required alignment of the wafer surface with the focal plane of the microlithographic projection lens. Not only is it important to control the distance of the wafer surface from the lens but it is also becoming increasingly important to level the surface of the wafer with respect to the lens' axis. Further, such leveling is preferably performed at each image site on the wafer since the surface of the wafer itself may not be flat within the tolerances required. While various independent optical systems have been devised for separately measuring distance to a wafer surface and its local tilt, these separate systems have not been readily applicable to microlithographic projector designs where the projection lens must extend into close proximity with the wafer surface.
Among the several objects of the present invention may be noted the provision of novel tilt sensing apparatus; the provision of such apparatus which combines tilt sensing and distance measurement; the provision of such apparatus which can be utilized in conjunction with a microlithographic projection lens which extends close to the surface of a wafer being printed; the provision of such apparatus which is highly accurate; the provision of such apparatus which can operate in real time during the manufacture of integrated circuits; the provision of such apparatus which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.